1. Field of the Invention
The present invention generally relates to image forming methods and apparatuses and, more particularly, to an image forming apparatus, such as an image setter or a plotter for producing a layout paper or a printing plate technique, which can correct a connecting portion error between images formed by adjacent writing units.
2. Description of the Related Art
Conventionally, there is an image forming apparatus, such as disclosed in Japanese Laid-Open Patent Application No. 9-185196 (hereinafter, referred to as Patent Document 1), which comprises a plurality of recording heads for recording images on a recording medium attached to a drum, a beam spot position detector such as a PSD detector and recording head moving means such as a linear motor, so as to perform an irradiation position correction to the drum by adjusting positions of the recording heads in accordance with information regarding the detected beam spot position to correct a connecting portion of the images formed by adjacent recording heads.
Additionally, Japanese Patent Publication No. 3067942 (hereinafter, referred to as Patent Document 2) suggests a method of correcting a positional misalignment at a connecting portion of images formed by adjacent image forming apparatuses. In this method, one or more image forming apparatuses delays formation of images of a first column in a head moving direction until a reference line drawing image forming apparatus completes scanning corresponding to a number of columns, which is a rounded number of positional misalignment columns with respect to one or more image forming apparatuses having a positional misalignment in a head moving direction greater than a width of a column in a moving direction of a recording medium by using, as a reference line, one of a plurality of image forming apparatuses. Further, all the image forming apparatuses completed the initial column scanning, the arrangement of the image forming apparatuses is moved by a distance smaller than a width of the columns so that the image forming apparatuses repeatedly form the same image as a preceding column to overlap with the preceding column, and continues it until the rest of the columns are completely scanned.
According to the apparatus of the Patent Document 1, costs of mechanical parts and a control system of the apparatus are increased since independently controllable moving means must be provided to a plurality of recording heads. Additionally, the cost is further increased if a number of the heads is large.
Additionally, according to the method of the Patent Document 2, an image quality cannot be improved because a line of dark color is emphasized, which is easily visible, or a column having a width close to a width of two columns is formed, which is more easily visible than the boundary since the columns of images formed two timed by using the same data are overlapped in a case where there is dispersion in an amount of misalignment in adjacent image forming apparatuses since the same distance smaller than a width of column is shifted uniformly irrespective of the amount of misalignment of the adjacent image forming apparatuses and image formation is performed using the same image data as a preceding column after all of the image forming apparatuses formed an image of a first column.
In the meantime, an image forming apparatus has an energy irradiation device that irradiates energy. The energy irradiated by the energy irradiation device is converged onto a printing plate material on which an image is formed. The printing plate material is caused to be changed due to this energy. Positions of the printing plate material and the energy irradiation device can be relatively moved. For example, the printing plate material is applied onto an inner surface or an outer surface of a rotatable drum, and the energy irradiation device is arranged on a table that is movable in a direction parallel to the rotational axis of the drum. Accordingly, the printing plate material is movable in a vertical direction due to a movement of the drum, and in a horizontal direction due to a movement of the table. A flux of energy of the energy irradiation device can be located at an arbitrary position on the printing plate material by a rotation of the drum and a movement of the table. Thus, an image original data is recorded as changes in a surface of the printing plate material by the energy irradiation device scanning the surface of the printing plate material while causing drawing point arrangement of the image original data to be formed and position of the printing plate material to correspond to each other and relating concentration information of the drawing point of the image original data to the output energy of the energy irradiation device.
The speed at which an image is recorded on the printing plate material is determined by items such as a rotational speed of the drum, a sensitivity of the printing plate material to energy, an energy that can be irradiated by the energy irradiation device, a recording density of the image, etc. Since the printing plate material on which an image is to be formed is normally an object, other items may be adjusted according to the sensitivity of the printing plate material. The sensitivity of the printing plate material is represented by a degree of change in the printing plate material and the energy density given at that time. The rotational speed of the drum and the irradiation energy of the energy irradiation device are determined as an energy density to the printing plate material. If the rotational speed of the drum is increased while the irradiation energy is constant, the energy density to the printing plate material is decreased correspondingly. On the other hand, if the irradiation energy is increased while the rotational speed of the drum is constant, the energy density to the printing plate material is increased.
Moreover, it is necessary to increase the rotational speed of the drum by increasing the irradiation energy when it is desired to increase a recording speed of an image. In order to increase the irradiation energy, there are a method of increasing a maximum output of an energy source and a method of providing a plurality of energy sources. As an example of the later, there is a method, as disclosed in the above-mentioned Patent Document 2, a method of placing a plurality of energy irradiation devices on the table. According to this method, energy fluxes of the energy irradiation devices are irradiated onto the printing plate material. The energy irradiation devices record an image in a form of sharing the printing plate material in the axial direction of the drum. The energy fluxes of the plurality of energy irradiation devices are arranged linearly.
However, it is difficult to arrange the energy irradiation apparatuses completely linear form. That is, the energy fluxes cannot be arranged in a linear form accurately due to an error when fixing the energy irradiation devices on the table, an error in positions at which the energy fluxes are converged, an error in positions where energy is generated, etc.
Due to the above-mentioned errors, an image recorded on the printing plate material cannot be one in which the image original data is reflected. This is because there is a misalignment generated between the positions of the image original data shared by the energy irradiation devices and positions actually recorded on the printing plate material. This misalignment is recognized in a from of an overlap of images between the energy irradiation devices, a gap generated at a boundary between the energy irradiation devices, or two horizontal lines formed by a single horizontal line having a step at a boundary between the energy irradiation devices.
In order to avoid such a misalignment in images, an adjustment is made so that lines indicating the same horizontal position are recorded on a single line also on the printing plate material by advancing or delaying a time of start of drawing by each of the energy irradiation devices using one of the energy irradiation devices as a reference. Additionally, when images are recorded in an overlapping manner, there is a method in which a start of image formation of one of the overlapping energy irradiation devices is skipped by a time corresponding to the overlap.
However, a positional accuracy of image overlapping between printing plates is an important quality control item for multi-color printing to obtain a desired function by overlapping a plurality of sheets of printing plate images or a wiring pattern of an electronic circuit board. The printing plate material has an expandability of a certain level, and an environmental condition such as temperature or humidity and image writing energy may be a cause of deformation. In such as case, an accuracy of overlapping printing plates can be improved by matching a previously formed printing plate so as to form an image. Additionally, when forming a single image by energy irradiation devices, each of the energy irradiation devices performs image formation simultaneously. Accordingly, a misalignment may be generated due to an error in positions of mounting the energy irradiation devices. If an attempt is made to cause a misalignment at a boundary to be less visible according to an amount of feed in a sub-scanning direction, a misalignment at other boundaries may become large since all of the energy irradiation devices moves simultaneously.
In the meantime, as a conventional image recording apparatus, there are a direct recording type, which performs image recording directly on a recording medium such as a recording paper and an indirect recording type, which performs image recording on an intermediate recording medium and transferring the image to a final recording medium. There is a laser printer or the like using an electrophotography process among image recording apparatuses of the indirect recording type which is widely used presently. In the laser printer using an electrophotography process, a latent image is formed by exposing a photosensitive material which has been charged according to recording image information and, thereafter, visualizing it by a developer and transferring the developed image onto a recording medium such as a recording paper so as to record the image. In this method, the image to be recorded is formed as a form of a latent image on the photosensitive material which is an intermediate recording material. As means for forming image information on the photosensitive material which is an intermediate recording material, a laser exposure device, which scans a laser light by a rotating polygon mirror, is mainly used. Additionally, as other means for forming image information on the photosensitive material, there are known an LED array exposure device, in which many LEDs are arrange in an array form, and a liquid crystal shutter array exposure device, which performs an exposure control by arranging liquid crystal shutters in an array form.
On the other hand, as the method of directly recording an image on a recording medium such as a recording paper, there are methods put into practice such as an inkjet recording method, which perform jet recording by injecting ink directly onto a recording paper according to image information, a thermal recording method, which records an image by directly contacting a heat generating array to a heat-sensitive recording medium, a thermal transfer recording method, which causes a thermal transfer ink sheet to contact a recording medium and transfers ink onto the recording medium by a heat-generating material.
In order to obtain a recorded image of a large size according to the image forming method using the above-mentioned laser exposure device, a width of the laser must be enlarged. Thus, a large distance must be provided between the polygon mirror and the photosensitive material which is an intermediate recording material, and, thereby, the size of the laser exposure device is increased. Additionally, if a scan width is large, a canning speed is increased, which causes a clock of the image to be recorded to be fast. For these reasons, the laser exposure device is rarely used for an image recording apparatus for a large size such as exceeding A3-A2.
On the other hand, according to the image forming method using an array head such as an LED array device or a liquid crystal array, an array head having a length corresponding to a recording width must be used so as to obtain a recorded image of a large size. However, since a high-resolution of more than 300-600 dpi is required for an array head of an image printing apparatus, it is not easy to elongate an array head manufactured using a semiconductor process. Additionally, since the electrophotography process is a page recording method, an array head corresponding to a recording width is required.
On the other hand, according to the recording method to cause a recording head itself to move and scan in a main-scanning direction such as an inkjet recording method, a thermal recording method and thermal transfer method, an image recording apparatus, which can attain image recording of a large size, can be relatively easily constructed. However, according to the recording method causing a recording head to scan, a moving mechanism of the head is complicated, and there is a problem in that a recording time is increased as a size of an image to be recorded is increased. In order to solve such as problem, it is effective also for the method to cause a recording head to scan to use an array head of a long scale corresponding to a recording width. However, it is difficult to fabricate a long scale array head. Further, in order to obtain a long scale recording head for a recording head of an LED array exposure device or the like, it is needed to fabricate array recording heads each having a certain length by using a semiconductor process and connecting the recording heads to each other with high accuracy. In such a case, a recording image width of A3 size requires 300 mm or more and a recording image width of A4 size requires 420 mm or more. Thus, it is difficult to fabricate a recording head of a long scale by a semiconductor process alone.
In the meantime, generally, in an image recording apparatus, it is required to have a recording density of 300-600 dpi or more, that is, a recording head element density of 80-40 μm or less. However, it is difficult to fabricate a head having such a high recording density by connecting recording head elements, and it will become extremely expensive recording head. Additionally, a pitch fluctuation may occur easily at connecting position between the recording head elements. Thus, there is a problem in that the connecting position tends to be easily observed as an image fluctuation in the recorded image.
Moreover, the above-mentioned Patent Document 1 suggests a method of correcting an error between head elements in a recording apparatus which is capable of recording an image of a large size by driving simultaneously a plurality of aligned recording heads, which irradiate beam spot lights onto a recording medium attached to a cylindrical drum, so as to detect an irradiated position of each of the spot lights.
In order to correct the error between the head elements like the method of the above-mentioned Patent Document 1, a correcting mechanism for mechanically adjusting positions of the head elements. However, it is difficult to achieve such a correcting mechanism satisfying the requirements of high accuracy. If the positions of the head elements are not adjusted accurately, there may be generated a step, a gap or an overlap in a formed image.